The present invention relates to a semiconductor design technology and, more particularly, to an internal voltage generator capable of providing a stable internal voltage for a semiconductor memory device even if an external voltage is unstable.
Generally, an internal voltage generator in a semiconductor memory device is a circuit to generate different internal voltages using an external voltage VDD. This internal voltage generator should generate a stable internal voltage regardless of the changes in temperature, processes or pressure.
FIG. 1 is a conventional internal voltage generator of a semiconductor memory device. Referring to FIG. 1, the conventional internal voltage generator includes a level detector 10 to detect a voltage level of a high voltage VPP, an oscillator 20 to produce a periodic signal OSC_OUT in response to a detection signal VPP_DET from the level detector 10, and a charge pump 30 to produce the high voltage VPP, which is higher than an external voltage VDD, by pumping charges from the external voltage VDD.
In the conventional internal voltage generator, a voltage drop in the high voltage VPP is detected by the level detector 10 and the oscillator 20 is then driven to produce the periodic signal OSC_OUT. The charge pump 30 maintains the high voltage VPP at a desired voltage level in response to the period of the periodic signal OSC_OUT.
The high voltage VPP generated as mentioned above is applied to an internal circuit block in the semiconductor memory device. At this time, an amount of current, which is supplied from the charge pump 30, should be larger than that which is consumed in the internal circuit block of the semiconductor memory device. If the amount of current which is consumed in the internal circuit block is larger than that which is supplied from the charge pump 30, a voltage drop in the high voltage VPP causes cell data to not be stored properly in a cell capacitor. Accordingly, the amount of current supplied from the charge pump 30 should be kept larger than that consumed in the internal circuit block.
When the different operations, such as an auto-refresh and a burst write/read operation, are carried out in the semiconductor memory device, a large amount of current consumption occurs in the internal circuit block and this large current consumption of the internal circuit block also causes a voltage drop in the external voltage VDD. That is, in the case where this instant current consumption of the external voltage VDD is excessive, the external voltage VDD undergoes a voltage drop.
As mentioned above, in the case where the external voltage VDD undergoes the voltage drop, since the external voltage, which functions as a driving power, is not sufficiently supplied to the internal circuit block, the high voltage VPP which is produced by pumping charges from the external voltage VDD also undergoes a voltage drop. Therefore, a malfunction can be caused at data read and write operations because of the voltage drop of the high voltage VPP. Furthermore, this voltage drop can be a problem in low-voltage and high-speed semiconductor memory devices, such as a graphic DRAM, which use a low external voltage and operate at high speed. Also, in the case where the external voltage VDD has a high level, the amount of current that needs to be supplied from the charge pump 30 is increased in proportion to the increase of the external voltage VDD. In this case, unnecessary current consumption is caused and an excessive increase of the high voltage VPP may cause an overshooting.
As mentioned above, when the voltage level of the external voltage VDD fluctuates, the amount of current for the high voltage VPP also fluctuates. This makes the actual high voltage VPP insufficient or excessive, which may be illustrated by referring to the following equation 1.Q+I*Δt=C*V I=C*V/Δt  [Equation 1]
As shown in the above equation, the amount of current supplied from the charge pump 30 can be expressed as a function of the external voltage VDD, the capacitance (C) of the charge pump 30 and the time (the period of the periodic signal OSC_OUT, Δt). Here, the capacity has a constant value which is determined by a circuit designer, but is limited to an appropriate value because a larger capacitor causes an increase in chip size. Further, the periodic signal OSC_OUT of the oscillator 20 has a constant period even if the external voltage VDD is fluctuated (a variation can be caused by skew and temperature changes, but this variation may be ignored).
The amount of current supplied from the charge pump 30 is changed based on the external voltage VDD. For example, even though the external voltage VDD comes from a DC power source, the voltage level of the external voltage VDD can be changed based on the amount of current consumption in the internal operation of the semiconductor memory device. At this time, an amount of current supplied from the charge pump 30 is also changed according to the fluctuation in the voltage level of the external voltage VDD. That is, even if the amount of current consumed in the internal circuit block is constant, the amount of current supplied from the charge pump 30 can be reduced when the external voltage VDD is decreased. Further, when the external voltage VDD is increased, the amount of current supplied from the charge pump 30 is also increased and then the high voltage VPP is increased. As shown in the above equation, the reason why this fluctuation in the voltage level is caused is that the amount of current supplied from the charge pump 30 is in proportion to the voltage level of the external voltage VDD.
Therefore, in case of the conventional internal voltage generator, since the periodic signal OSC_OUT of the oscillator 20 has a constant period regardless of the voltage level of the external voltage VDD, the amount of current supplied from the charge pump 30 can be decreased and the high voltage VPP can undergo the voltage drop. When the high voltage VPP is decreased, the reliability of the cell data, which are stored in memory cells and read out from the cells, deteriorates because the word line to be driven by the high voltage VPP cannot obtain a sufficient voltage level.
Also, when the external voltage VDD is increased, the increase of the high voltage VPP may cause an overshooting because the amount of current supplied from the charge pump 30 is increased.